The present invention relates generally to radio frequency (RF) power amplifiers for spread spectrum digital cellular telephones, and more particularly to such an amplifier having an output stage biased for class AB operation to achieve a linear replica of a spread spectrum input signal of the amplifier. Another aspect of the invention relates to a bias circuit for such an amplifier, wherein circuitry suppresses a tendency of the bias circuit to oscillate in a manner to affect operation of the amplifier.
Mobile telecommunication systems using spread spectrum techniques, such as code division multiple access (CDMA) and third generation (3G), can accommodate a large number of users per given bandwidth. In one particular type of CDMA system, known as direct sequence CDMA (DS-CDMA), a base station applies to data signals a pseudo-random noise binary sequence selected for a particular cellular phone, i.e., mobile station. The cellular phone responds to the sequence to derive a sequence synchronized with the base station sequence to establish a communication link between the cellular phone and base station. Power of the link is varied as a function of the power requirements to achieve satisfactory communications between the base station and cellular phone. For example, when the phone is far from the base station, the link power is greater than when the phone and base station are close to each other.
FIG. 1 is a block diagram of some elements of a prior art CDMA cellular telephone 10. Data source 11, e.g., a voice source or tone sources responsive to keyboard activation, drives signal processor 12 for processing the signal of source 11 into a phase modulated, spread spectrum signal that drives power amplifier 16 which in turn drives conventional transmitter 17, including a switch or diplexer, coupled to antenna 18.
Processor 12 includes a data modulator, carrier generator, spread spectrum modulator, and a pseudo-random binary sequence generator, all configured in a known manner. The data modulator of processor 12 responds to signals from a CDMA base station in proximity to telephone 10 to vary the amplitude of the spread spectrum signal the processor supplies to power amplifier 16 as a function of the power needed to provide adequate transmission between telephone 10 and the CDMA base station. For example, if there is a strong transmission link between telephone 10 and the base station because they are close to each other, the amplitude of the signal driving amplifier 16 is substantially less than the amplitude of the signal driving the amplifier if there is a weak transmission link because the base station and telephone are far apart. Battery 20 supplies power to all components in CDMA phone 10, including power amplifier 16.
A parameter associated with radio frequency power amplifiers is xe2x80x9cpower added efficiency,xe2x80x9d defined as the amplifier output radio frequency power (PRFO) minus the amplifier input radio frequency (PRFI) divided by the DC power supplied to the amplifier (PDC), i.e.,                     P        RFO            -              P        RFI                    P              D        ⁢                  xe2x80x83                ⁢        C              .
To achieve the desired power added efficiency, the output stage of the prior art power amplifier of FIG. 1 is biased to operate in class AB mode; i.e., a power amplifying transistor of the power amplifier of FIG. 1 is biased to be on more than 180 degrees and less than 360 degrees of each cycle of the amplifier input signal. As a result of the prior art amplifier being off during a portion of each cycle of the amplifier input, the amplifier linearity decreases, i.e., the amplifier output signal is not a faithful linear replica of the amplifier input signal. The amplifier is usually designed so there is a compromise between power added efficiency and linearity, with the optimum combination being achieved by setting the highest power added efficiency possible for a given linearity requirement. In CDMA cellular telephones, the limit for the power added efficiency is set according to the linearity requirement for the adjacent channel power ratio, i.e., the ratio of power in a channel which is one channel away from the main signal channel (the distortion product) to the power in the main signal channel. Meeting the linearity requirement requires proper control of the quiescent current in the power amplifier.
Our co-pending application Ser. No. 09/621,525, filed Jul. 21, 2000, herein incorporated by reference, discloses a cellular telephone power amplifier with control of power transistor quiescent current using an impedance-controllable biasing circuit. This amplifier achieves high power added efficiency while maintaining substantial linearity by biasing a control electrode of the transistor and controlling quiescent current flowing through the power amplifying transistor to maintain the power transistor in class AB operation.
We found the power amplifier linearity was improved by adding a low pass filter to circuitry for biasing the control electrode. The low pass filter, comprising a shunt capacitor and series resistor, reduces the alternate-channel-power ratio, i.e., the ratio of power in a channel which is two channels removed from the main signal channel to the power in the main signal channel. The reduced alternate-channel-power ratio resulted in improved linearity. This improvement in linearity reduced power amplifier harmonics and noise and leads to greater power added efficiency.
Our co-pending application Ser. No. 09/730,657, filed Dec. 6, 2000, herein incorporated by reference, discloses a cellular telephone power amplifier with a power transistor biasing circuit including a low pass filter having a shunt capacitor that acts as a self bias booster. The low pass filter, in the preferred embodiment, includes a 15 ohm series resistor and a 2.2 picofarad capacitor, resulting in the filter having a 33 picosecond (33xc3x9710xe2x88x9212 s.) time constant. With this bias arrangement, the power amplifying transistor is biased to operate in class AB mode.
While such circuitry was effective in improving power amplifier linearity, we have found that it is possible to improve linearity even more without materially adversely affecting power added efficiency. It is accordingly an object of the present invention to provide a new and improved cellular telephone linear power amplifier without excessive compromise of power added efficiency.
Experiments with the biasing circuitry of the ""525 application reveal that the biasing circuitry has a tendency to oscillate at frequencies or harmonics which affect the operation in a pass band of the cellular telephone power amplifier. Oscillations or harmonics resulting from this tendency interfere with and can possibly mask proper transmission from the cellular phone to the base station.
It is, accordingly, another object of the invention to provide a new and improved cellular telephone with a biasing circuit having a tendency to oscillate so as to interfere with the signal transmitted from the phone, wherein the tendency is suppressed.
In accordance with one aspect of our invention, a mobile telephone of a spread spectrum wireless telephone system has a radio frequency power amplifier comprising (1) a power transistor including a control electrode for controlling a variable impedance path between first and second electrodes of the transistor, and (2) a bias circuit for supplying a bias voltage to the control electrode such that the transistor operates in class A or AB mode as a function of the spread spectrum signal amplitude at the control electrode. The bias circuit includes a low pass filter having a time constant between about 0.15 microseconds and 1.5 microseconds. Time constants in this range are thus about 10,000 to 100,000 times the time constant of our prior art circuit, disclosed in the ""657 application. The substantial increase in time constant apparently maintains the power transistor control electrode bias constant regardless of the level of the r.f. signal applied to the electrode, and enables the power transistor to operate in class A for the low amplitude r.f. signals and in class AB for the high amplitude r.f. signals. Thus, higher power added efficiency is provided for the high amplitude signals. The increase in low pass filter time constant provided by the present invention over the prior art has an overall salutary effect on the power amplifier and cellular phone operation. Preferably, the low pass filter includes a series resistor having a resistance of about 15 ohms to control the current flowing to the control electrode and a shunt capacitor having a capacitance in the range of 0.01 to 0.1 microfarads.
In accordance with another aspect of the invention, a mobile telephone of a spread spectrum wireless telephone system includes a radio frequency power amplifier comprising (1) a power transistor having a control electrode responsive to the spread spectrum signal and controlling a variable impedance path between first and second electrodes of the transistor, and (2) a bias circuit for supplying a bias voltage to the control electrode. The bias circuit includes a low pass filter causing (1) the transistor to operate in class A or AB mode as a function of the amplitude of a spread spectrum signal at the control electrode, and (2) the power amplifier to derive an output signal that is a substantial replica of the spread spectrum signal for all amplitudes of the spread spectrum signal.
In accordance with yet another aspect of the invention, a mobile telephone of a spread spectrum wireless telephone system includes a bandpass radio frequency power amplifier comprising (1) an amplifying transistor including a control electrode responsive to the spread spectrum signal and controlling a variable impedance path between first and second electrodes of the transistor, and (2) a bias circuit for supplying a bias voltage to the control electrode. The bias circuit includes first and second transistors connected together in a feedback circuit, which tends to oscillate to have an effect on the operation of the power transistor. Circuitry, preferably in the form of a low pass filter, coupled to the feed back circuit suppresses the tendency for the oscillations to be coupled to the power transistor.
The features and advantages of the present invention will become apparent upon consideration of the following detailed description of a specific embodiment thereof, especially when taken in conjunction with the accompanying drawings.